I host a small gathering of gamers and non-gamers at my place occasionally. We're always trying to find games that we can get as many people involved in as possible. Without question, Towerfall Ascension is the fan favorite of the group. We've poured hours into it, with everyone (regardless of gaming pedigree) clambering to grab the controller from the clutches of the most recent defeated.
Gameplay is sharp and dynamic enough to sate the more experienced crowd, leading to some nail biting match ups that never feel cheaply won. However, the concept is simple and straightforward enough for a lay person to go from a prickly pincushion to a sneaky usurper in a few short play sessions. The matches are fast and chaotic enough that the game never gets boring to watch from the sidelines, especially if the controller is changing hands a lot.
TL:DR if you have a group of people looking for a fun, pick up and play, game- Towerfall Ascension definitely deserves a look!